Aqueous phase polymerization of ethylene

ABSTRACT

ETHYLENE IS POLYMERIZED BY CONTACTING AND AGITATING GASEOUS ETHYLENE WITH AN AQUEOUS SOLUTION CONTAINING A LOW CONCERTRATION OF A FREE RADICAL INIATOR AND A NONIONIC SURFACTANT IN A CONCENTRATION BELOW ITS CRITICAL MICELLE CONCENTRATION, AT A TEMPERATURE BELOW 100*C. AND UNDER A PRESSURE OF ABOUT 10 TO 100 ATMOSPHERES FOR A PERIOD OF TIME OF ABOUT 1/2 TO ABOUT 8 HOURS. THE MOLECULAR WEIGHT OF THE PRODUCT INCREASES WITH INCREASING PERIODS OF POLYMERIZATION. POLYETHYLENE HAVING A HIGH DEGREE OF LINEARITY, HIGH PRUITY AND A NARROW DISTRIBUTION OF MOLECULAR, WEIGHT OVER THE RANGE OF ABOUT 50,000 TO 350,000 IS OBTAINED BY THE PROCESS.

United States Patent 3,629,224 AQUEOUS PHASE POLYMERIZATION 0F ETHYLENEEnrico Cernia and Arturo Rio, Colleferro, Rome, and Natale ErcoliMalacari and Corrado Mancini, Milan, Italy, assignors to Societa AsfaltiBitumi Cementi e Derivati, S.p.A., Rome, Italy No Drawing.Continuation-impart of application Ser. No. 436,645, Mar. 2, 1965. Thisapplication June 3, 1969, Ser. No. 830,111

Claims priority, application Italy, Mar. 10, 1964, 5,195/ 64 Int. Cl.C08f 1/60, 3/06 US. Cl. 260-949 A 7 Claims ABSTRAIZT OF THE DISCLOSUREEthylene is polymerized by contacting and agitating gaseous ethylenewith an aqueous solution containing a low concentration of a freeradical initiator and a nonionic surfactant in a concentration below itscritical micelle concentration, at a temperature below 100 C. and undera pressure of about to 100 atmospheres for a period of time of about /2to about 8 hours. The molecular weight of the product increases withincreasing periods of polymerization. Polyethylene having a high degreeof linearity, high purity and a narrow distribution of molecular weightover the range of about 50,000 to 350,000 is obtained by the process.

This is a continuation-in-part of our copending US. patent applicationSer, No. 436,645 filed Mar. 2, 1965.

BACKGROUND OF THE INVENTION This invention relates to a new process forthe polymerization of ethylene by means of a free radical initiatorwhich leads to polymers of high molecular Weight and high crystallinity.In particular, the present invention concerns the polymerization ofethylene in the presence of an aqueous medium.

The polymerization of ethylene in the form of an aqueous emulsion toproduce normally solid polyethylene was first proposed by Hopff et al.in 1956. However, it proved to be extremely difficult to obtainemulsions having the necessary ethylene concentration. Furthermore,emulsions of ethylene in aqueous media are inherently unstable andrequire particularly vigorous agitation for their preservation evenunder high ethylene pressure.

The polymerization of ethylene in solution in various solvents has alsobeen proposed, for example in US. Pat. No. 2,334,195. In such processes,as in the polymerization of ethylene in an emulsion, it is difficult toobtain solutions of ethylene at high, stable ethylene concentrations.This is exceedingly disadvantageous; the limiting factor with respect tothe rate of ethylene polymerization in solution is the relatively lowspeed of the dissolution of ethylene in the solvent. However, even ifthe problems of solubility and diffusion did not exist, the chaintransfer mechanism of polymerization in solution would be detrimental tothe quality of the resultant polymer.

For these reasons, processes for the polymerization have been proposed,as in US. Pat. No. 2,475,628, in which ethylene is polymerized in thegaseous state in the presence of a homogeneous gas phase catalyst toproduce normally solid polyethylene. Such a polymerization is carriedout with good yield at a temperature lower than the softeningtemperature of the resultant polymer. However, although thepolymerization of ethylene in the gaseous phase has many advantages overpolymerization in solution or emulsion, there is one problem, which, ifnot overcome prevents the practical application of this process. Thepolymerization of ethylene in the gaseous phase produces a polymer whichstratifies on the walls of the reactor.

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Thus, the actual volume of the polymerization reactor is progressivelyreduced in a relatively short time to such an extent, that it isnecessary to interrupt the process of polymerization in order to removethe polymer present in the reactor. Furthermore, under these conditions,it is impossible to control the reaction, nonhomogeneous products maythereby be obtained, and in addition there is a serious danger ofexplosion.

To overcome these difficulties, it has been proposed to line the wallsof the reactor with a continuous film of water containing a surfactant.It was observed that ;although the water did not impede the accumulationof the polymer on the walls of the reactor, the addition of surfactantsprevented the polymer from adhering to the walls, probably due to anaction of preferential absorption on the part of the surfactant on thewalls themselves. In this process, the polymer is not mechanically sweptfrom the walls of the reactor, but it is not permitted to come intocontact with them. For this purpose, a thin film of water, relativelyslow moving and containing a surfactant is sufficient.

More recently, an improvement in the process of ethylene polymerizationhas been proposed in US Pat. No. 2,728,755 using as catalysts, theperoxide carbonate esters, which have also been described in US. Pat.Nos. 2,475,- 628; 2,475,643; and 2,475,648 for the polymerization ofethylene, and a liquid, separate carrier phase to remove the polynierproduct from the reaction area. The liquid carrier phase in this processneither acts as a solvent nor forms an emulsion in the reaction area andit is therefore readily separable together with the product from theunreacted ethylene and catalyst. The product-carrying liquid used ingreat quantities, contains in solution a surfactant and prevents thepolymer product from being deposited on the walls of the reactor. Inthis process, the polymerization takes place in the dense gaseousethylene phase and the polyethylene formed is then transferred as adispersion into the separate carrying phase whereupon it is removed fromthe reactor.

SUMMARY OF THE INVENTION A principal object of the invention is theprovision of an improved process for polymerizing ethylene in thepresence of an aqueous medium and at a temperature below the softeningtemperature of the resultant polymer whereby polyethylene of uniform andimproved quality is obtained.

Another object of this invention is the provision of a process ofpolymerizing ethylene in which the polymerization takes place in theaqueous medium, whereby the exact conditions desired of temperature andpressure may be maintained and whereby the development of thepolymerization may be controlled by varying the parameters of thereaction.

-A still further object of this invention is the provision of a procesfor the polymerization of ethylene in which the reaction mixturecontinues to polymerize even after the complete disappearance of thecatalyst and wherein both the conversion rate and the molecular weightand structure of the resultant polymer may be regulated by the period oftime during which the polymerization is carried out.

These objects and others are accomplished by the present invention whichwill be described below.

It has been found that polyethylene of high purity, having a narrowdistribution of molecular weight in the range of about 50,000 to 350,000is produced by polymerizing ethylene in the presence of an aqueoussolution containing a low concentration of a free radical initiator anda surfactant in a concentration belows its critical micelleconcentration.

According to the present process, ethylene is agitated with an aqueoussolution containing a free radical initiator in an amount of about 0.001to 0.5 percent by weight based on the weight of ethylene and asurfactant in a concentration below its critical micelle concentrationat a pressure of about 100 to 1,000 atmospheres and a temperature notabove 100 C. The molecular weight and structure of the polyethyleneproduct, as well as the rate of conversion of ethylene to polyethyleneare dependent upon the period of time that the polymerization ispermitted to proceed; therefore, the reaction is discontinued when aproduct having the desired molecular weight is obtained.

In addition to having uniform high molecular weights, polyethyleneproduced according to the present process has a very high chainlinearity, the methyl content being less than 7 per 1,000 carbon atomsand an elongation percentage at break of over 500 percent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the presentprocess, a surfactant must be present in the aqueous solution in whichethylene is posurfactant in a concentration below its critical micelleconcentration. The critical micelle conentration, as discussed inFrontiers in Colloid Chemistry, by R. E. Burk and Oliver Gummit,published by Interscience Publishing Company, 1959, refers to the lowestconcentration of a surface active agent at which an emulsion is formedor the colloid micelles begin to grow. Below the critical micelleconcentration no emulsion is formed.

Depending on the particular surfactant used, and also the propertiesdesired in the final polymer, in the present process a surfactant isgenerally present in the aqueous medium in a concentration of about0.001 to 0.5% by weight. The concentration of surfactant used accordingto the present process is more often in the range of 0.01% to 0.2% andmost preferably 0.05% to 0.3% based on the weight of water.

As will be discussed below, polymerization according to the presentprocess takes place initially solely in aqueous solution, formingcolloidal aggregates of low molecular weight polymer. Desirably, thesurfactant should be capable of increasing the rate of dissolution ofethylene in water. Also the surfactant should provide suitableconditions for the formation and stabilization of the colloid. For thepurpose of the present process, the surfactant should have alyophilic-lyophobic balance, i.e. [2 (total number of the lyophilicgroup)-Z (total number of the lyophobic grups) +7], such that thelyophobic groups allow the transfer of the polymerization locus from theaqueous to the colloidal phase, and the lyophilic parts cause initiallythe stabilization of the colloidal aggregates and subsequently of thepoly/monomer particles.

Nonionic surfactants are used in the present process; those having thedesired characteristics include, for example, Lubrol W, Aionico LNS andAntarox Co 630, alkyl aryl polyethyleneglycol, alkyl polyethyleneglycoland products of esterification of saturated and unsaturated fatty acidsof long and short chains, with the sorbitan where the hydroxylicfunctions of sorbitan have been condensed with varying amounts ofethylene oxide. The following surfactants are also used in the presentprocess: products of condensation of fatty amines with ethylene oxide(Araphen K 100, Araphen T 100, Noramox S 3, Noramox S 7, etc.) that is,nonionic products of Weak cation active character which can form ofsalts with long chain organic acids and are therefore modifiable intheir lyophilic character. Especially preferred surfactants include thecondensation product of the nonylphenol with 15 moles of ethylene oxide,the condensation product of the higher alcohols, such as lauryl andcetyl alcohol with 20 moles of ethylene oxide, the polyoxyethylenesorbitan monostearate which contains 20 moles of ethylene oxide, amixture 6040 by weight of sorbitan monolaurate and polyoxyethylenesorbitan monolaurate, and the condensation product of 7 moles ofethylene oxide with a primary amine containing 16 carbon atoms. Thesematerials may be used alone or as mixtures.

A higher alcohol, that is an alcohol containing about 14 to 20 carbonatoms, such as cetyl alcohol may be employed together with thesurfactant in the present process in a concentration of about 0.001 to0.2% by weight, based on the weight of water.

According to the present invention, the initiator is a radical typeinitiator and must have a brief transformation time and a lowdecomposition energy in order to be active at the temperature at whichthe reaction is conducted. Among the various initiators having thenecessary characteristics, alkyl peroxycarbonates and some azo compoundswhich have proved to be particularly suitable for this type of processare used.

In a preferred embodiment of this process, a mixture of dialkylperoxycarbonate and an tit-substituted acylperoxide is used as theinitiator. Among the dialkyl peroxidicarbonates used in the presentprocess may be mentioned diethyl peroxidicarbonate, diisopropylperoxidicarbonate, di(tertiarylbutyl) peroxidicarbonate and diisoamylperoxidicarbonate. Examples of suitable u-substituted acylperoxidesincludes a-chloro-dibenzoylperoxide and a,ot'-dichlorodibenzoylperoxide.

The initiator is present in the aqueous medium in a concentration ofabout 0.00005% to 0.4%, preferably 0.007% to 0.03%. The amount ofinitiator present relative to the concentration of ethylene in theaqueous phase which is about 0.001% to 5% strongly influences the courseof the reaction. According to the present invention, the initiator ispresent solely in the aqueous phase. Under ordinary conditions theinitiator may be only partiallysoluble in water at the desiredconcentration, however the presence of the surfactant causes theinitiator to be completely dissolved in the aqueous medium.

The present process can be carried out at a temperature from about 10 toC., dependent on the type of polymer desired and also dependent on thestability of the surfactant. Each surfactant has a decompositiontemperature above which it loses its activity. At low temperatures, thepolymerization reaction is extremely slow. The optimum temperature forcarrying out the polymerization of ethylene according to the presentprocess is between 60 and 75 C.

The optimum pressure of the present reaction is about 500 to 600atmosphere; however, the reaction can be conducted at a pressure ofabout 100 to 1,000 atmospheres. Most preferably, the pressure ismaintained at a constant value during the polymerization, generally,additional ethylene is fed into the reaction as necessary for thispurpose.

In view of the fact that the amount of ethylene converted and themolecular weight of the polyethylene product varies depending with theperiod of polymerization when polymerization is carried out according tothe present process, it is believed that the present process ischaracterized by three reaction stages. However, it is understood thatthe invention is not limited by any theoretical considerations.

In the first stage of polymerization, the initiator which is dissolvedin the aqueous medium decomposes with the formation of free radicals,thus initiating the polymerization of the ethylene dissolved in thewater. There is no initiator present in the gaseous phase, and hencepolymerization takes place only with respect to the ethylene dissolvedin the aqueous medium. The concentration of ethylene in the aqueoussolution is, of course, dependent upon the temperature and pressureunder which the polymerization is carried out. However, the approximaterange of the solubility of ethylene is of the order of 0.6 and 0.7 gramof ethylene per 100 grams of Water. A more complete discussion of thesolubility of ethylene in water may be found in an article by E. J.Bradbury et al., Industrial and Engineering Chemistry, vol. 44, No. 1,page 211. Due to the high proportion of free radical concentration toethylene monomer concentration in the first stage of the reaction, alarge number of polymer molecules of limited size, i.e. low molecularweight and of a particular structure are obtained.

The prepolymer contains a balance between lyophobic and lyophilic groupsso as to form a new surfactant in situ; the lyophilic groups arise fromthe decomposition of the catalyst and/or chain transfer with thesurfactant, and the lyophobic group is constituted by the shortpolyethylene chain. The concentration and structure of the prepolymer isdependent upon the parameters of the reaction, i.e particular initiatorchosen, concentration of initiator, temperature and pressure.

The second stage of the polymerization may be considered to be in thecolloidal phase, since the prepolymer or new surfactant produced in situtogether with the original surfactants can form the so called compositemicelles, i.e. colloidal aggregates stabilized by surfactant. In thisstage, wherein there are numerous low molecular weight polymericmolecules of such structure that they are easily situated among, andprobably surrounded by surfactant molecules, the solubility of theethylene monomer is increased. Consequently, there is an autoacceleration of the rate of polymerization, which in turn results in thegrowth of the polymer molecules. In this way, the colloidal aggregatesare transformed into solid stabilized particles which form the new andprincipal locus of polymerization in the third stage of the reaction. Thus, macroradicals mainly constituted by the colloidal aggregatesoriginate during the second stage of polymerization.

In the third and final polymerization stage, a heterophase reactionoccurs between the polymer particles and monomer diffused therein. Thus,the polymer molecules continue to grow depending on the amount ofdiffused monomer in the macromolecules.

Vigorous agitation should be used, in order that the monomer be in closecontact with the locus of polymerization. Suoh agitation can be effectedin various ways, for example beating, bubbling, turbulence, rotation etcetera. It has been found that circular agitation, in which the reactionmixture is drawn from the bottom of the reactor and then dropped downagain from above in a finely divided spray by mechanical means, or theaction of a compressed gas such as, in particular, ethylene itself is anespecially desirable procedure for obtaining the necessary contactbetween ethylene and the polymer particles. Suitable gas diffusors mayalso be used to attain the desired close contact between the gaseous andaqueous phases.

One of the important characteristics of the present process is theliving mechanism" of the third stage of the polymerization reaction,wherein the presence of long lived radicals, referred to herein asmacroradicals provide polymers of particularly high molecular weight.Further, as a result of the presence of the macroradicals which arepresent in the preparation of polyethylene in accordance with thepresent invention, the molecular weight of the product increases withincreasing periods of polymerization.

In accordance with the present process, after an interval of time, asthe polymerization proceeds, there is a continuous, substantial decreasein the ratio between the concentration of the initiator and theconcentration of monomer. In the third stage of polymerization, theproduction of free radicals decreases, since most of the initiator hasalready been expended. Termination by the usual mechanisms, namelycoupling and disproportionation diminishes notably with thedisappearance of initiator, and ceases entirely when the decompositionof the initiator is complete. The macromolecules tend to be buried inthe solid phase, and direct termination of polymerization betweenmacromolecules which might occur as a result of the collision thereof onthe surface is extremely improbable. The high viscosity of the mediumalso makes termination reactions between buried macroradicalsexceedingly improbable.

The low degree of chain transfer in the present medium is also animportant factor in obtaining a polymer product of high molecularweight. Thus, in the third stage of the reaction, the only mechanism oftermination of polymerization is that of polymer-polymer and polymermonomer chain transfer. However, in the present process, the mechanismof chain transfer is low and there is an enormous difference in theoverall rate of initiation and the rate of termination.

Thus, in the present process, the macroradicals are permitted to matureor live during a relatively long reaction time; therefore the presentprocess cannot be carried out in the stationary state of a continuousprocess.

Another advantageous characteristic of the present process, is that thehighly viscous medium of the third stage of the polymerization leads tothe continuous immobilization of radical tails, which results in apolymeric chain having a high degree of linearity.

Water is used as the dispersing medium because chain transfer mechanismsare absent or negligible therein. Water is also a desirable medium forthe present reaction due to its high thermal capacity and its low cost.

The high thermal capacity of water serves as an internal heat stabilizerfor the present reaction and provides for the dissipation of anyelevation in the heat of reaction and therefore lends itself to theexact control of the reaction temperature. In practice, generally about4 to 8% by weight of ethylene based on the volume of water is used inthe present process.

The present process may be carried out in a plant in the followingmanner.

Ethylene, in a concentration higher than 99.9% and containing less than20 p.p.m. of oxygen is compressed by the pressure of exercise and isthen fed in parallel to a series of reactors which operate out of phasein time. After polymerization has been carried out for the desired time,the reaction mixture is unloaded cyclically at ambient atmospheric orslightly higher pressure (connected to an ethylene gasometer); thismixture contains a certain amount of water which is maintained betweenand C. The combined action of flashing which occurs at the time ofunloading and of steam distillation almost entirely eliminates theethylene contained in the polymer.

The water in which the polymer is maintained as a dispersion by means ofthe action of the surfactant and/or agitation is ultimately expelledfrom the polymer by filtration which can be effected in the usual meanssuch as a centrifuge.

The water of the first filtration which contains a high concentration ofsurfactant is stored and used for further reactions. The polymer is thensubjected to a series of washings with pure water in order to eliminateas far as possible any impurities Subsequently, the damp slurry comingfrom the filtration and containing between 10 and 30 percent of water ispartially dried to about 2 percent of humidity, preferably using apneumatic drying system with hot gas such as nitrogen in a closed cycle.The polymer dust is then finally dried, most preferbaly by being placedinto a rotating cylinder with hot gas such as nitrogen so that thehumidity is decreased to about 0.2%. The polymer dust which has beendried in this way is sent to a storage bin after passing through aseries of vibrating sieves where it undergoes the final finishing phase,namely granulation and sacking.

In comparison with traditional high pressure processes, the presentprocess which is considered to be a medium pressure process issubstantially advantageous in having a lower operative and investmentcost, due to the use of simpler apparati and to the reduction in thecosts of compression. In comparison with the low pressure process, thepresent process is advantageous in the elimination of solvent leaks andin the absence of metallic impurities in the finished polymer.

Also, the use of water as the reaction medium is exceeding advantageousin that reaction heat is eliminated without the undesirable use of anorganic solvent as a heat stabilizer commonly used in the low pressureprocesses.

Further, the polymer product obtained according to the present processhas the desirable characteristics of linearity found in the productsobtained by the low pressure process together with the purity andhomogeneity of the high pressure process. Another advantageous featureof the present process is the ease of obtaining polyethylene in a widerange of molecular weights by simple regulation of reaction parameters,especially the period of polymerization.

The following examples further illustrate the best mode contemplated forcarrying out the present process; however these examples must not beconsidered as limiting the concept or scope of the present invention inany manner whatsoever.

EXAMPLE 1 Water, surfactant (0.050% based on the weight of water) andisopropylperoxicarbonate (0.014%) are introduced at room temperatureinto a stainless steel autoclave, provided with a heating jacket and arotating agitator capable of producing a high number of revolutions persecond, and supplied with a suitable device which permits a high rate ofexchange between the liquid and gaseous phases. After having made aninert gas bubble, the autoclave is closed and then fed with ethylene inan amount of by weight based on the weight of water. In a period of -15minutes, the system is brought to 65 C. and reaches the fixed pressureof 500 kg./cm. A pressure fall, which begins after a brief period ofinduction, is compensated by continuous and regular reloading whichthereby permits operation at a constant pressure.

After a reaction time of 1 hour, the autoclave is degassed. The productappears as a fine suspension of polyethylene in Water which is easilyseparated by filtration. The polymer dust obtained in this way is oncemore dispersed in water, filtered and dried. A conversion of 22%(considering the ethylene consumed for the recharging) is obtained.

The characteristics of the polyethylene obtained are as follows:

Molecular weight 55,000

Density (gr./cc.) 0.9518

CH /1000 C 6.46

Color Very white Ashes Absent EXAMPLE 2 Polymerization is carried out inexactly the same manner as described in Example 1, except that thereaction time is prolonged to 2 hours. In this case the followingresults are obtained:

Conversion 30% Molecular weight 95,000 CH /1000C 5.71

EXAMPLE 3 Polymerization is carried out under the same conditions asExample 1, except that the reaction time is increased to 4 hours. Thefollowing results are obtained:

Conversion 38% Molecular weight 140,000 RCH:CH /1000 C 0.310

EXAMPLE 4 Polymerization is carried out under the same conditions asExample 1, except that the reaction time is prolonged to 8 hours. Thefollowing results are obtained:

Conversion (percent) 45 Molecular weight 182,000

RRC=CH /1000C 0.041

EXAMPLE 5 The autoclave described in Example 1 is loaded with 1800 p.w.of water without oxygen, 0.4 part of diethylperoxicarbonate and 5.6parts of a nonionic surfactant (a product of the condensation of a longchain fatty alcohol and oxide of ethylene). The autoclave is closedunder an ethylene load and then charged with ethylene containing lessthan 20 ppm. of oxygen. The total quantity of ethylene being g. so thatthe proportion of ethylene is 4.4% based on the volume of water. Thereaction mixture is heated to 65 C. and kept for 2 hours at 50() kg./cm.The reactor is then cooled, freed from excess gas, and opened. Thepolyethylene dust obtained is washed with water and dried in a vacuumstove. Twenty four parts of polymer are obtained, characterized by amolecular Weight of 55,000 and a density of 009410.

The repetition of the above example, using only 1.4 parts of the samesurfactant, gives 48 parts of polyethylene having a molecular Weight of118,000 and a tensile strength of 200/cm.

EXAMPLE 6 A cylindrical stainless steel autoclave, furnished with amixer, is filled to 4/5 with an aqueous solution to 0.1% of a nonionicsurfactant. Then the diisopropylperoxicarbonate is added, in aconcentration of 0.007% with respect to the solution. After the removalof the air, the ethylene is compressed up to a pressure of 280 kg./cm.The autoclave is then heated to a pressure of 500 kg./cm. This pressureis maintained for 3 hours with a maximum variation of 5 kg./cm. Finally,after degassing, polyethylene dust is obtained which is filtered anddried without undergoing further treatment. Thirty seven grams ofpolymer are obtained with a molecular weight of 110,000 and a Vicatsoftening point of 117 C.

Polymerization is carried out in the same manner, changing only theconcentration of the initiator (0.028% of water) after 3 hours, 64 gramsof polymer with a molecular weight of 52,000 are obtained.

EXAMPLE 7 Two 1. of solution formed by 0.20% of a nonionic surfactantand 0.014% of t.butylperoxicarbonate is poured into an autoclaveprovided with a suitable stirring system. After extracting the air, 123grams of ethylene which reach a pressure of 400 l g./cm. at 65 C., arecompressed in the autoclave. Keeping this pressure constant bycontinuous recharges, for a period of 2 hours, after degassing,filtration and drying, 25 grams of polyethylene dust with a molecularweight of 50,000 and a density of 0.9455 are obtained.

This polymerization is repeated in the same manner except that 135 gramsof ethylene are introduced at a temperature of 65 C. and a pressure of600 kg./cm. 55 grams of polymer with a molecular weight of 139,000 and adensity of 0.9432 are obtained.

EXAMPLE 8 One hundred twenty nine grams of gaseous ethylene are added to2 l. of the same solution as that in Example 7. The system is heated to65 C. and reaches a pressure of 500 kg./cm. The reaction, undercontinuous agitation, is left to proceed under falling pressure. After 8hours the pressure is 252 kg./cm. After the normal filtration and dryingtreatments, 57 grams of polymer dust with an intrinsic viscosity inDecalin at C. of 1,600 dl./gr. are obtained.

EXAMPLE 9 In a tubular, verticle, stainless steel reactor, furnishedwith a device which permits the continuous circulation of the reagentsand products of polymerization from the bottom to the top of the reactoritself, 1500 parts in weight of water are introduced under weak ethylenepressure. By means of a heating jacket, this is brought to a temperatureof 80 C. (reactor temperature). At this point, a feeding pump introduces3 parts of a nonionic surfactant and 0.42 of an alkylic peroxicarbonateemulsified in a little water. Lastly, more ethylene is introduced untilthe desired pressure of 500 kg./c1n. is reached. A recycle of 1000 partsof reagent per minute is passed through the gaseous phase in acontinuous manner, in this way assuring the gas-liquid diffusion and theagitation of the system. After a reaction time of 120 minutes, at aconstant pressure through continuous reloading, 124 parts of polymerdust characterized by a molecular Weight of 135,000 and a density of0.9420 were obtained.

A similar reaction, held for the same length of time at 60 C., produced80 grams of polymer in a very fine powder form with a molecular weightof 175,000 and a slightly higher density.

EXAMPLE 10 A vertical tubular reactor is filled 2/ 5 with a solution of0.1% of a surfactant. In the reactor it is possible to Withdrawcontinually from the head, part of the gaseous phase only, which is thenreplaced by means of a centrifugal compressor, at the bottom of thereactor, in this way the double effect of diffusion of the gaseous phaseand the agitation of the medium is obtained. The initiator,isopropylperoxicarbonate, is added in a quantity of 0.028% to the water.When the temperature of the solution has reached 70 C. the ethylene iscompressed to a pressure of 300 kg./cm. and the centrifugal compressoris set in motion. After a period of 100 minutes the gaseous phase iseliminated, obtaining a suspension of polymer in water which isfiltered, treated with steam to eliminate the excess of surfactant,separated by centrifugation and finally dried with nitrogen at a hightemperature. Fifty three grams of polymer were obtained with a meltindex of 1.84 gr./.

EXAMPLE 11 Polymerization is carried out as described in Example 1except that the reaction temperature was 60 C. and as the initiatorequal quantities of isopropylperoxicarbonate anda-chloro-dibenzoylperoxide are used in a total concentration of 0.020.The reaction is allowed to proceed for 2 hours. The results of thepolymerization are:

Conversion (percent) 34 Molecular Weight 76,000 Density (gr./cc) 0.9531CH /1000C 5.00

What we claim and desire to secure by Letters Patent 1. A process forthe polymerization of ethylene to produce polyethylene of high purityand linearity and having a narrow distribution of molecular weight ofabout 50,000 to 139,000 which comprises introducing ethylene to anaqueous solution containing a water soluble free radical initiator in anamount of about 0.007% to about 0.03% by weight based on the weight ofwater and a nonionic surfactant in a concentration of about 0.05 to 0.3%based on the weight of water which is below its critical micelleconcentration, the ethylene being introduced in an amount of 5% byweight based on the weight of the water, agitating the ethylene togetherwith said aqueous solution, at a temperature of about 75 C. and apressure of to 1,000 atmospheres, said initiator being present only inthe aqueous phase and thereby initially polymerization takes place onlywith respect to ethylene dissolved in the aqueous solution and varyingthe period of time for polymerization from about /2 hour to about 8hours to obtain a product of the desired molecular Weight, the molecularWeight increasing with an increasing period of polymerization.

2. Process according to claim 1 in which said free radical initiator isa dialkyl peroxycarbonate.

3. Process according to claim 1 in which ethylene is added during thepolymerization to maintain the pressure at a constant value.

.4. Process according to claim 1 in which the pressure during thereaction is maintained at 50060 atmospheres.

5. Process according to claim 1 in which the reaction mixture isagitated by the continuous circulation of the aqueous mixture as adispersion through the gas phase.

6. Process according to laim 1 in which the part of the gaseous ethylenephase is continually Withdrawn from one zone of the reaction andreplaced in another zone of the reactor in contact with the aqueousphase thus effecting the diffusion of the gaseous ethylene in theaqueous phase and the agitation of the mixture.

7. In a process for the polymerization of ethylene at an elevatedtemperature and a pressure of about 500 to 600 atmospheres in thepresence of an aqueous solution containing a nonionic surfactant and awater soluble free radical initiator to produce polyethylene having amolecular Weight of about 50,000 to 139,000, the improvement whichcomprises introducing ethylene to said aqueous solution in an amount of5% by Weight based on the Weight of water, said aqueous solutioncontaining about 0.007% to 0.03% by weight of initiator and 0.05 to 0.3%of surfactant, the initial polymerization of ethylene taking place onlywith respect to ethylene dissolved in said solution and then varying thetotal period of time for the polymerization from about /2 hour to about8 hours to obtain a product of the desired molecular weight, themolecular weight increasing with increasing periods of time.

References Cited UNITED STATES PATENTS 2,685,577 8/1954 Cerveng et al.260-94.9 2,728,755 12/1955 Weisemann 26094.9 2,788,340 4/1957 Dannels260-94.9 3,089,865 5/1963 Walther et al. 260--87.1 3,119,802 1/1964Guillet et al 260-949 3,496,157 2/1970 Cernia et al 26094.9

FOREIGN PATENTS 1,429,582 l/ 1966 France.

JOSEPH L. SCHOFER, Primary Examiner E. J. SMITH, Assistant Examiner

